Nitric oxide has a multitude of effects in living tissues. The mechanism of these effects is nearly always based on interaction of nitric oxide either with a metal component (typically iron) or with thiol groups of key enzymes and other proteins. Depending on the particular enzyme, such interaction can lead to either activation or inhibition of the protein. An example of an effect based on the activation of an enzyme is that of vasodilatation: nitric oxide binds to the haem iron of the enzyme guanylate cyclase, which results in conformational change exposing the catalytic site of the enzyme. This leads to catalytic conversion of GTP to cGMP. This conversion initiates the whole cascade of reactions leading to protein phosphorylation and muscle relaxation (vasodilatation). Other effects based on activation of enzymes or growth factors by nitric oxide include stimulation of cell division (proliferation) and cell maturation, stimulation of cell differentiation and formation of cell receptors, neovascularisation, formation of fibroblasts in the wound and thereby enhancement of collagen formation, etc.
Topical delivery of nitric oxide can be a very useful feature in various therapeutic or cosmetic applications including wound healing, treatment of skin or nail infections, sexual dysfunction etc.
Under normal conditions, nitric oxide (NO) is a short-lived, unstable gaseous substance. Its instability is due to the unpaired electron of nitrogen. It is therefore beneficial to deliver nitric oxide topically in the form of a nitric oxide donor which diffuses into the body site and releases nitric oxide, either spontaneously or on activation. Particularly useful nitric oxide donors are nitrosothiols. Nitrosothiols are donors of nitric oxide which can be released by their spontaneous decomposition:2R—SNO→2NO+R—S—S—R
The rate of decomposition varies considerably depending on the side chain of the thiol. For example, whilst nitrosocysteine can be almost totally decomposed within minutes under normal conditions, it takes hours/days to achieve almost complete decomposition of nitrosoglycerol. The decomposition can be accelerated markedly in the presence of Cu2+ and Hg2+. Nitrosothiols are also able to donate nitric oxide directly onto another thiol group. This process, which is called trans-nitrosation, is quite common in vivo:R1—SNO+R2—SH→R1—SH+R2—SNO
Nitrosothiols can be produced by reaction between thiols and nitrite in an acidic environment. The reaction mechanism involves formation of nitrosonium cation (NO+) which, in turn, reacts with a thiol to produce corresponding nitrosothiol:NO2−+2H+→NO+H2OR—SH+NO+→R—SNO+H+
S-nitrosothiols can thus be produced conveniently by mixing a thiol (e.g. thioglycerol) with a source of nitrite (e.g. potassium nitrite) in acidic solution. The reaction proceeds at pH<6, the rate of the reaction increasing with the acidity of the solution:R—SH+NO2−+H+→R—SNO+H2O
WO2006/095193 discloses a skin dressing adapted, on activation, to release one or more S-nitrosothiols. The dressing comprises one or more components containing a source of nitrite, a source of thiol and a source of protons.
It is a well known fact that the rate of the nitrosothiol generation can be controlled by pH. In principle, the rate increases with increasing acidity of the formulation containing a source of nitrite and a thiol. However, whilst it is possible to ensure such rapid generation of nitrosothiols simply by adjusting pH, the acidity required may prevent applicability of such formulation (for example when applied onto intact or wounded skin).
Whilst some applications may require only a slow rate of nitrosothiol generation, there are other applications that benefit from a rapid burst of nitrosothiols. It is possible to ensure such rapid generation of nitrosothiols simply by adjusting pH, but the acidity required may prevent applicability of such formulation (for example when applied onto intact or wounded skin).